1. Field of the Invention
This invention relates to lens systems for projecting an image onto a focal plane and more particularly to the field of optical scanning systems for periodically and reciprocally translating an image on an image plane between a first and second limit position while maintaining high image quality on a flat image plane. The invention optical scanning system is particularly adapted for use in light weight portable FLIR (Forward Looking Infrared) surveillance systems or imaging systems used for missile guidance and is typically employed with an objective lens system operating between the invention scanner and object space. The objective lens system is adapted to converge the light rays passing from object space, the light rays then passing through the invention scanner and being focused on a relatively flat image plan.
2. Description of the Prior Art
Various optical scanning systems are shown in the art for deflecting or scanning a scene image across an image plane or sensitive focal plane. These devices range from a simple mirror or refractive element rotating about an axis perpendicular to the optical axis of the system to other more sophisticated systems such as those using counter-rotating wedges as described in the "Fundamentals of Optics" by Jenkins and White, McGraw-Hill, 1957, pages 23 and 24. The counter-rotating wedge scanner is widely accepted in the optics field for applications requiring precise control of angle of deflection and beam control.
One problem associated with some optical scanning systems known in the art is that they translate the image on the image plane with a sinusoidal velocity on the image plane. This problem is overcome to some extent by optical scanning systems that utilize non-sinusoidal driving mechanizations. Counter-rotating wedge prism scanners are sometimes used to generate a straight line scan from a point light source. A disadvantage of the counter-rotating wedge prism scanner is that the counter-rotating wedge beam scanner wedges must be rotated in opposite direction with perfectly matched angular velocities. Variations in the respective angular velocities of each of the respective wedge prisms will result in a deflection error producing an orbital or elliptical motion of the image on the image plane.
Another problem associated with counter-rotating wedge beam scanners is the requirement for precision in the manufacture of the respective rotating wedges for those applications requiring relatively constant image velocity on the image plane. Another limitation of the counter-rotating wedge scanner when used to scan an image on an image plane with relatively constant image velocity on the image plane is low optical efficiency. As the wedges are counter rotated, the image translates on the image plane with a velocity varying as a sinusoidal function. To linearize the image velocity on the image plane, the application must restrict the usable range of the scan from typically minus thirty degrees to plus thirty degrees of prism rotation for each half cycle of prism rotation. The image, or the signals derived from the image on the image plane, are blanked or remain unused during the remainder of the counter-rotating wedge prism angle of rotation. The efficiency is therefore reduced by a factor defined by the ratio of blanking time to half cycle rotation time.
An oscillating flat mirror can be used with a lens system to form an optical scanning system; however, such systems require beam folding. Another prior art system of interest uses a rotating polygon having mirrored flat surfaces; however, such systems produce a focal point that describes an arc or elipse as the polygon rotates.